Cavity Formation in a SiC/SiC Composite under Simultaneous ...

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Cavity formation was investigated in a SiC/SiC composite under multi-ion beam irradiation up to 10 dpa at 1073 K, 1273 K and 1573 K by transmission electron ...
Materials Transactions, Vol. 46, No. 3 (2005) pp. 536 to 542 Special Issue on Fusion Blanket Structural Materials R&D in Japan #2005 The Japan Institute of Metals

Cavity Formation in a SiC/SiC Composite under Simultaneous Irradiation of Hydrogen, Helium and Silicon Ions Shuhei Miwa1; * , Akira Hasegawa1 , Tomitsugu Taguchi2 , Naoki Igawa2 and Katsunori Abe1 1 Department of Quantum Science and Energy Engineering, Tohoku University, 01 Aramaki-aza-Aoba, Aoba-ku, Sendai 980-8579, Japan 2 Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki 319-1195, Japan

Cavity formation was investigated in a SiC/SiC composite under multi-ion beam irradiation up to 10 dpa at 1073 K, 1273 K and 1573 K by transmission electron microscopy. The cavity formation behavior of each component of the composite was dependent on the component’s grain structure, the helium and/or hydrogen implantation mode, and irradiation temperature. It was found that helium rather than hydrogen is likely to enhance cavity formation or cavity swelling. The contributions of helium and hydrogen to cavity formation in the composite components are discussed in detail. (Received September 21, 2004; Accepted February 9, 2005) Keywords: SiC fiber reinforced SiC matrix composite, triple ion beam irradiation, transmission electron microscopy, helium, hydrogen

1.

Introduction

Silicon Carbide (SiC) has been proposed as a candidate structural material for fusion reactor applications due to its low activation and high strength at higher temperatures. SiC/ SiC composites have been developed using advanced SiC fibers to improve their mechanical properties and thermal conductivity.1,2) Previous studies have examined the neutron irradiation resistance of SiC/SiC composites, consisting of stoichiometric SiC fibers with a low oxygen content and high crystallinity, such as Hi-Nicalon Type S and Tyranno SA.3) However, in addition to displacement damage, helium (He) and hydrogen (H) are produced in SiC by nuclear transmutation reactions initiated by 14 MeV neutrons. The maximum gas production rate in SiC at the first wall region was estimated to be approximately 2000 at.ppm He/(MWa/ m2 ) and 800 at.ppm He/(MWa/m2 ) in the ARIES IV blanket design.1) The effects of these gaseous elements in SiC are summarized as follows: helium is insoluble in SiC and likely stabilizes vacancy-type clusters produced by displacement damage, thereby enhancing cavity formation.4–6) The solubility of hydrogen in SiC is also very low. Hydrogen is trapped at both Si- and C-sites by the displacement damage, where it forms C-H or Si-H.7) Thermal desorption spectrometry has shown that the mobility of hydrogen in SiC increases above 1073 K.8) Therefore, hydrogen could potentially enhance cavity formation at temperatures of 1073 K and above, which corresponds to the operating temperature range of a fusion reactor using a SiC/SiC composite. The effects of the simultaneous production of displacement damage, helium and hydrogen in SiC must be known to predict the material’s lifetime in the fusion reactor environment. The purpose of this study is to clarify the synergistic effects of He and/or H on cavity formation in the SiC/SiC composite matrix and fiber components. *Corresponding

author, E-mail: [email protected]

2.

Experimental

The two-dimensional SiC/SiC composite with Hi-Nicalon Type-S fibers examined in this work was fabricated at Oak Ridge National Laboratory (ORNL).9) The SiC fiber weaves in the composite were coated with pyrolytic carbon (PyC) by chemical vapor deposition (CVD) to prevent the SiC fiber from uniting with the SiC matrix. The resulting thickness of the PyC coating was approximately 150 nm. The -SiC matrix was deposited onto a SiC fabric lay-up by forced-flow chemical vapor infiltration (FCVI). Simultaneous multiple ion beam irradiation with Si, He and H was performed in the TIARA (Takasaki Ion Accelerators for Advanced Radiation Application) facility of JAERI (Japan Atomic Energy Research Institute). The accelerated energies were 6.0 MeV for Si2þ , 1.0 MeV for Heþ and 0.34 MeV for Hþ . Energy degraders were used to obtain broad depth distributions of He and H along the displacement damage distribution produced by the Si-ion irradiation. Figure 1 shows the calculated depth distribution of the implanted Si, He and H, and the displacement damage

Fig. 1 The depth distribution of displacement damage, He, H and Si in SiC calculated using TRIM code.10)

Cavity Formation in a SiC/SiC Composite under Simultaneous Irradiation of Hydrogen, Helium and Silicon Ions

in SiC calculated with the TRIM code10) using displacement energies of 35 eV (Si) and 20 eV (C). The irradiation dose in the area examined was approximately 10 dpa. Irradiation temperatures were 1073 K, 1273 K and 1573 K. Several combinations of simultaneously irradiated ions were used including Si (Single), Si + H (Dual H), Si + He (Dual He), Si + He + H (Triple) and Si + He + H  10 (Triple H  10) in which the H concentration was ten times higher than in the Triple-irradiation. He and H concentration to dpa ratios were 130 at.ppm He/dpa and 40 at.ppm H/dpa (400 at.ppm H/dpa for Triple H  10 irradiation). Microstructural observations were carried out using a transmission electron microscope (TEM, Hitachi HF-2000) at 200 keV. TEM samples were prepared using a Focused Ion Beam machine (FIB, Hitachi FB-2000A) at the JAERI Tokai Laboratory. Cross-sectional observations were performed on the SiC fibers, the SiC matrix and the PyC coating layer. Cavity formation behavior was studied by measuring the mean cavity diameter and the number density of the cavities. In the SiC matrix of the SiC/SiC composites, the spatial distribution of cavities in the irradiation region was determined by observation of cavity density in a meshed region of 100 nm  100 nm. 3.

Results

Figure 2 presents TEM micrographs of typical regions within the SiC matrix for Single-, Dual He-, Triple- and Triple H  10-irradiation at 1073 K, 1273 K and 1573 K. Figure 3 presents TEM micrographs of typical regions within the SiC fiber for Dual He-, Triple- and Triple H  10irradiation at 1273 K and 1573 K. Table 1 shows the mean cavity diameter and the number density of cavities in regions within the SiC matrix and SiC fiber that were irradiated simultaneously. In all these observations of the SiC/SiC composites, dislocation loops were not observed in the SiC matrix or the SiC fibers. In neutron-irradiated monolithic CVD-SiC, many dislocation loops have been observed,11) and H. Kishimoto et al. reported the formation of dislocation loops within monolithic CVD-SiC for single-ion beam irradiation.12) From these previous studies, dislocation loops were thought to form within the SiC matrix and the SiC fiber. However, the fine grain size of the SiC matrix and the SiC fiber compared to the monolithic CVD-SiC and the strong contrast from stacking faults in SiC prevented the imaging of dislocation loops. 3.1 Cavities in the SiC matrix (1) Single-irradiation Figures 2(a), (e) and (i) show the typical microstructure of the Single-irradiated SiC matrix at 1073 K, 1273 K and 1573 K. At 1070 K, cavities were not detected after Singleirradiation. At 1273 K, however, cavities were observed only near the carbon coating layer, ranging to approximately 400 nm, and throughout the irradiation region at 1573 K. Figure 2(i) shows the typical cavity distribution observed at grain boundaries and stacking faults in the matrix at 1573 K. (2) Dual H-irradiation The cavity distribution for Dual H-irradiation was almost

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the same as that for Single-irradiation, as shown in Table 1. These results demonstrate that the effect on cavity nucleation of H alone, with coexisting displacement damage, was ultimately small under these irradiation conditions. (3) Dual He-irradiation Many cavities were observed at grain boundaries at 1073 K, 1273 K and 1573 K, and relatively smaller cavities were also observed at stacking faults at 1573 K as shown in Figs. 2(b), (f) and (j). Compared with the results of Singleirradiation, an enhancement of cavity formation at grain boundaries was observed for Dual He-irradiation. (4) Triple-irradiation Figures 2(c), (g) and (k) show the typical cavity distribution due to Triple-irradiation. Cavities were observed at grain boundaries at 1073 K, 1273 K and 1573 K. Smaller cavities were also observed at stacking faults at 1573 K. Compared to the effect on the SiC matrix of Dual He-irradiation, an enhancement in cavity formation at grain boundaries was observed for Triple-irradiation at 1073 K and 1273 K,13) as shown in Table 1. At 1573 K, however, an enhancement of cavity formation in excess of that due to Dual He-irradiation was not clearly detected. (5) Triple H  10-irradiation Figures 2(d), (h) and (l) show the typical cavity distribution for Triple H  10-irradiation. Many cavities were also observed at grain boundaries at 1073 K, 1273 K and 1573 K. Smaller cavities were observed at stacking faults at 1573 K. Compared to the effects of Triple-irradiation, the number density of cavities increased at 1073 K, and the mean cavity size at 1273 K and 1573 K decreased somewhat, as shown in Table 1. (6) Overview of cavity formation in SiC matrix For the SiC matrix, various results were obtained in response to the irradiation temperature and observed region. The following are summaries of the cavity formation behavior of a SiC matrix: (a) The observed cavities were located mainly on grain boundaries at 1073 K and 1273 K, as shown in Fig. 2. Larger cavities were observed at grain boundaries, and smaller cavities were observed at stacking faults at 1573 K. This shows that cavity growth occurred preferentially at the grain boundaries rather than at stacking faults. (b) Cavity formation was observed for Dual He-, Tripleand Triple H  10-irradiations at above 1073 K. As shown in Table 1, the mean cavity diameter at grain boundaries increased with increasing irradiation temperature for the above-mentioned irradiation conditions. The number density of cavities at grain boundaries was higher at 1273 K than at 1073 K, and decreased at 1573 K. The highest number density of cavities occurred in the transgranular region at 1573 K. (c) The shape of the cavities also changed with increasing irradiation temperature; spherical at 1073 K, ovalshaped at 1273 K and extended amoeboid shapes along grain boundaries at 1573 K, as shown in Fig. 2. The smaller cavities at stacking faults were spherical in shape even at 1573 K. (7) Non-uniform cavity distribution in SiC matrix In the ion irradiation experiment, damage and implanted

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S. Miwa, A. Hasegawa, T. Taguchi, N. Igawa and K. Abe

Fig. 2 TEM micrographs of typical regions within the SiC matrix for Single-, Dual He-, Triple- and Triple H  10-irradiation at (a)(d) 1073 K, (e)(h) 1273 K and (i)(k) 1573 K, respectively. The arrows indicate cavities.

ions have the depth distribution shown in Fig. 1, and uniform irradiation is received at identical depths. Therefore, the spatial distribution of damage along the direction lateral to the irradiation surface is uniform in the same material. However, non-uniform cavity distribution was observed in the irradiated region of the SiC matrix in this study. The spatial distribution of the cavities in the SiC matrix for Dual He-irradiation at 1073 K, 1273 K and 1573 K is shown in Fig. 4. The left side of the figures corresponds to the irradiation surface of the specimen. The vertical axis is the

distance from the interface of the SiC matrix and the carbon coating layer. Black/white contrast indicates the number density of cavities. The region of cavity formation sites depends on the irradiation temperature. In Fig. 4(a), cavities were observed only in the dual irradiation region at 1073 K. Figure 4(b) shows a non-uniform spatial distribution of the cavity number density in the SiC matrix near the carbon coating layer. In the vicinity of the carbon layer, the observed cavity area spreads along the implantation direction. The distance of the cavity formation area outside of the He and H

Cavity Formation in a SiC/SiC Composite under Simultaneous Irradiation of Hydrogen, Helium and Silicon Ions

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Fig. 3 TEM micrographs of typical regions within the SiC fiber for Dual He-, Triple- and Triple H  10-irradiation at (a)(c) 1273 K and (d)(f) 1573 K. The arrows indicate cavities.

Table 1 The mean cavity diameter and the number density of cavities in the simultaneous irradiation region within the SiC matrix and the SiC fiber. mean density [1022 m3 ]

mean size [nm] 1073 K matrix a Single



matrix b fiber matrix a

Dual H

matrix b fiber matrix a

Dual He

matrix b fiber matrix a

Triple

matrix b fiber matrix a

Triple H  10

matrix b fiber

none

1273 K 4:6  1:0 none

none none

none none

none 4:5  0:5 none

11:6  3:3

4:0  0:2 4:0  0:2

3:2  0:5

4:9  0:6 5:1  0:6 none 5:0  0:4 4:8  0:6 2:7  0:4 2:8  0:3 2:8  0:4 3:0  0:6

matrix: SiC matrix of SiC/SiC composite fiber: SiC fiber of SiC/SiC composite none: no cavity a : SiC matrix near carbon coating layer (